The prolongation of the action of subcutaneously administered insulin has obvious advantages in clinical practice, and the introduction of long-acting 'protamin-insulin' [Hagedorn, Jensen, Krarup, and Wodstrup, 1936] and 'zinc protamine-insulin' [Scott and Fisher, 1937] has been of great value in the treatment of diabetes mellitus. Experiments with the sex hormones have shown that when many of these substances are administered in the form of a tablet of solid material inserted under the skin, their physiological effectiveness is greatly increased, and their duration of action prolonged [Deanesly and Parkes, 1937]. The possibility that the subcutaneous administration of insulin in the form of tablets of solid material might result in an increased and prolonged action therefore seemed worthy of investigation. Methods Animals. Rabbits weighing 1·5–2·0 kg. were used in all the experiments recorded in this paper. They received an ample diet of cabbage, oats, and hay. Insulin. Commercial crystalline insulin containing 0·9%